The thermal design of electrodes of high pressure discharge lamps has become increasingly important in developing improved lamp designs. In a xenon-metal halide lamp, for instance, the thermal design of the anode electrode ("anode") and cathode electrode ("cathode") has become especially important. A xenon-metal halide lamp includes, in an arc chamber, a fill of metal and metal-halide substances promoting light generation, including xenon at a relatively high pressure, e.g. 6 atmospheres at room temperature. The xenon is stimulated to substantial light emission almost immediately upon energizing the lamp with a relatively high starting current. A typical starting current is 6 amps for about 3 seconds, for a 60 watt lamp. The starting current is followed by current ramping down to a considerably lower, steady state level of about 1 amp, for example, over the next 10 seconds, for a 60 watt lamp. Especially in the d.c. mode of operation, the high initial current causes especially pronounced heating of the anode, which should thus have a high heat capacity. The anode is also heated continuously during lamp operation in the process of receiving electron-current flow from the cathode during d.c. operation.
Further, especially where a xenon-metal halide lamp is vertically oriented, i.e. with its cathode positioned vertically above its anode, a molten metal halide pool typically covers about the lower third of the inside wall of the lamp, near the anode, during lamp operation. Efficient thermal management calls for the anode to be designed to facilitate heat radiation into the metal halide pool, to increase the halide vapor pressure, and thereby increase light output. To prevent the anode heat from being diverted down the supporting shank of the anode, the diameter of the anode shank can be minimized.
A prior art approach to thermally managing an anode of a xenon-metal halide lamp is to form the anode with a considerably larger mass than the cathode, and to machine the anode from a single, relatively large workpiece of refractory metal, by electric discharge machining, for instance. By so machining a single workpiece, a relatively large anode tip can be formed with a small diameter supporting shank, to minimize heat flow through such shank. Further, the outer surface of the anode tip can be textured in the machining process so as to increase the anode surface area available for radiating heat into the metal halide pool.
A shortcoming of machining an anode from a single workpiece in the foregoing manner is that the machining process is time-consuming and expensive. It would thus be desirable to provide a more economical method of making a high pressure discharge lamp with a thermally improved anode.